The invention relates to a fuel assembly for a pressurized-water reactor.
Several spacers are distributed over the length of the fuel rod to laterally fix the fuel rod bundle of a fuel assembly. On its underside, the fuel assembly has a foot piece, on the upper side of which the fuel rods are supported. A headpiece is provided at the top of the fuel assembly. If the fuel assemblies are installed in a reactor pressure vessel, they are disposed between a lower core grid and an upper core grid, fixing pins, which engage in bores at the foot piece and headpiece, projecting from the core grids. The core grids and the head and foot parts are usually produced from austenitic steel.
Two main materials are used for the spacers, namely austenitic steel and Zirconium alloy (xe2x80x9cZircaloyxe2x80x9d). Zircaloy contains tin, iron, chromium and optionally nickel, in addition to zirconium as its main constituent. Since austenitic steel has a relatively high neutron absorption, Zircaloy spacers are generally used. The two materials have different thermal expansions. The coefficient of thermal expansion of Zircaloy is approximately 5xc3x9710xe2x88x926 mm/mm xc2x0C., while austenitic steels have a coefficient of thermal expansion of approximately 18xc3x9710xe2x88x926 mm/mm xc2x0C.
Therefore, the spacers expand to a lesser extent than the core grids in the horizontal or radial direction, so that the gap spaces, which are present between the individual fuel assemblies, increase in size when heated to operating temperature.
However, a particular drawback of this is that the degree of freedom for lateral bending of the fuel assemblies (for example, caused by non-uniform pressure conditions) is increased. This may lead not only to the bending of an individual fuel assembly, but also the bending of all the fuel assemblies on one radius, (i.e., all the fuel elements disposed in a line within the inner wall of the core shroud).
Thus, deviations of 10-20 mm from the set position may arise (in the central region of the fuel assembly) as a result of the cumulative increased gap spaces. With fuel assemblies, which have been deformed in this manner, it is difficult for control rods to move into the guide tubes, and an emergency shutdown of the reactor may be delayed. Furthermore, there is a change in the moderation conditions which were originally present, which leads to undesirable power changes.
The degree of freedom of bending for a fuel assembly is increased not only by the different thermal expansion between spacer and core grid, but also by two further effects. A loading gap of approximately 1 mm is required between the fuel assemblies to allow safe loading and unloading of the reactor. In other words, the edge length or spacer pitch is smaller by this amount than the pitch of the core grids.
A further effect, which increases the size of the gap, results from the property of Zircaloy of growing under neutron irradiation. The growth may also be directed radially outward, so that, after prolonged neutron irradiation, the spacers have a larger pitch than in the starting state.
Although (after a prolonged operating period) the increase in the size of the gap space based on different coefficients of thermal expansion is partially compensated for in the operating state, a radially grown spacer would reduce the size of the loading gap (in the cooled unloading state of the reactor) and make it difficult to remove fuel assemblies which are to be exchanged. To avoid this, spacers with a reduction in pitch, which corresponds to the growth are provided from the outset. Therefore, the reduction in the pitch of the spacer of a fresh fuel assembly (i.e., during the initial operating phase in which noticeable growth of the spacers has not yet taken place) leads to an increase in the size of the gap spaces that are present between the fuel assemblies.
Conventionally, the approach taken to solve the above-problems was to configure the fuel assembly structure as torsionally rigid as possible. In particular, the wall thickness of the control rod guide tubes was increased, and the connection between the guide tubes and the spacers was reinforced.
It is accordingly an object of the invention to provide a fuel assembly for a pressurized-water reactor that overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and that counteracts an increase in the size of the gap space, which is present between the fuel assemblies which are installed in a reactor pressure vessel without requiring reinforcing measures for the fuel assembly structure.
With the foregoing and other objects in view, there is provided, in accordance with the invention, at least one spacer, which is preferably positioned in a central region of the fuel assembly, and is formed from a first part, which lies on the radially outer side with respect to the longitudinal center axis of the fuel assembly. A second, radially inner part is also provided. The second part is made of Zircaloy and the first part is made of a metallic material which, when compared to Zircaloy, has a lower growth in the radial direction, caused by neutron radiation and a higher coefficient of thermal expansion.
When compared with a conventional spacer (which is entirely made of Zircaloy), in the present invention, the outer, first spacer part expands to a greater extent when heated to operating temperature. The result is that the difference in thermal expansion between the spacer and core grid is reduced compared to the conventional configuration. The closer the coefficient of expansion of the material of the first part to the coefficient of thermal expansion of the core grid material, the more pronounced the reduction becomes.
If there is not a certain play between the first and the second part right from the outset, a play of this type is formed on account of the greater thermal expansion of the outer part. Therefore, radial growth of the inner part caused by neutron radiation is possible without any radial widening of the outer part. Thus, the growth of the inner part has no effect on the pitch of the spacer.
Accordingly, a reduction in the pitch of the spacer, which compensates for growth and increases the size of the gap spaces between the fuel elements, is not necessary for a fresh fuel assembly. Therefore, the initial gap space between the fuel assemblies can be limited to the loading gap. In this case, only a different thermal expansion between the core grids and the outer part of the spacer has the effect of increasing the size of the gap space.
However, if the outer part of the spacer is made of the same material as the core grids (e.g., austenitic steel), the extent to which the size of the gap space is increased as a result of heating is reduced. Therefore, the gap spaces between the fuel assemblies are limited substantially to the loading gap, even at the operating temperature.
In accordance with another feature of the invention, the first and second parts are separate parts, which are not fixedly connected to one another. Individual cells are disposed on the inner side of the outer part to ensure that the outer part is fixed to radially outer control rod guide tubes. The second part is then fixed to control rod guide tubes that lie further toward the inside. Thus, there is no need for a connection between the two parts.
In accordance with a further feature of the invention, the second part is formed from webs, which are fitted crosswise into one another. Several of the webs are composed of two partial webs, which are separated from one another in the axial direction (i.e., they leave an intermediate space between them, which extends over the entire web length).
An expansion web, which is formed of the same material as the first part, is provided in the intermediate space. The ends of the expansion web are fixed to the inner side of the first part. The first part is mechanically connected to the second part through this configuration. The expansion webs are dimensioned in such a way that a relative movement between them and the partial webs is possible. When heated to operating temperature, the expansion webs expand (so does the first part connected to them), so that the increase in the size of the gap between the fuel assemblies caused by the thermal expansion of the core grids is at least partially compensated for.
In accordance with an added feature of the invention (so as to use a material, which has the lowest possible neutron absorption (in particular austenitic steel)), the first part the first part is a frame formed of edge webs only.
In accordance with an additional feature of the invention, there is provided a positively locking connection acting in a longitudinal direction of a corresponding one of said edge webs.
Although the frame is connected to the second part via the expansion webs, which are interwoven into the grid of the second part to increase the mechanical stability of the spacer further, in accordance with a concomitant feature of the invention, the end regions of some of the webs and partial webs are secured against lateral tilting at the first part by a positively locking connection acting in the longitudinal direction of the respective edge web. Such a connection between the webs or the partial webs and the frame allows a relative movement between the webs or the partial webs and the frame in the longitudinal direction of the webs. Therefore, the frame can expand without obstacle when it is heated to operating temperature.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a fuel assembly for a pressurized-water reactor, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.